Roofs having single-ply roofing membranes are so named to contrast them with another group of commercial roofing products known as built-up roofing. Built-up roofs are constructed on the roof by the contractor using component materials such as felts and asphalt. As such, they are subject to the problems caused by weather, worker error, and material inconsistencies. Single ply membrane roofing systems using EPDM (ethylene propylene diene monomer) rubber, polyvinylchloride (PVC), and other synthetic material single layer sheets as the top layer of water impervious material were introduced on a large scale to the roofing industry several years ago. The single ply membrane roofing systems are used for flat or low pitched roofs, such as are found in most large commercial buildings such as office buildings, shopping centers and the like. The use of such single ply membranes is increasing due to their inherent advantages over older systems, such as built up roofs formed of layers of tar and paper material, because of weathering and the lower roof loading weights and the savings in costs for an installed roof. Single-ply membranes are flexible sheets of compounded synthetic materials that are manufactured in a factory to strict quality control requirements that minimize the risks inherent in built-up roof systems. Primary among the many physical and performance properties these materials provide are strength, flexibility, and long-lasting durability. The inherent advantages of prefabricated sheets are the consistency of the quality of the products that are manufactured, the versatility in their attachment methods, and therefore, their broader applicability. They are inherently flexible, used in a variety of attachment systems, and compounded for long lasting durability and watertight integrity for years of roof life.
Single-ply roofing membranes include thermoplastic membranes that are based on elastomeric polymers that may be processed as plastics. The most common thermoplastic is PVC, which has been made flexible through the inclusion of plasticizers. Thermoplastic membranes are identified by seams that are formed using either heat (i.e., RF welding or hot air) or chemical fusion (using solvent-borne cements). The resulting seams are as strong as or stronger than the membrane itself. Many thermoplastic membranes are manufactured to include a reinforcement layer such as polyester or fiberglass to provide increased strength and dimensional stability.
PVC membranes have also been used in other applications, such as geotextile membranes. A disadvantage to using PVC in roofing and other applications is their relatively high permeability to organic liquids. A method to reduce permeability of PVC membranes is to include another high-barrier polymer (e.g., polyamides and ethylene vinyl alcohol copolymers) in the membrane.
One approach is to prepare a multilayer membrane structure. The multilayer structure is made of at least two discrete, continuous layers of different polymers, including PVC as at least one of the layers, and is prepared by processes such as coextrusion, extrusion coating, lamination, and the like. This approach requires multiple extruders and often multiple operations to build up the structure, leading to higher costs.
Another method is to prepare melt blends of the different polymers. In many cases, the high-barrier polymer is not fully compatible or miscible with the PVC and heterogeneous blends result. In a heterogeneous blend, one of the polymers forms a continuous phase and the other polymer forms discrete regions or particles distributed throughout the blend (dispersed phase). If the PVC is the continuous phase and the amount of the high-barrier polymer is low, pathways for organic materials to permeate through the PVC phase may exist in the membrane. Accordingly, such blends require relatively large amounts of the high-barrier polymer in order to provide adequate barrier performance.
U.S. Pat. No. 5,352,735 discloses melt blends of halide polymers such as polyvinylchloride and polyamide at a temperature of no greater than about 220° C. and wherein the polyamide has a processing temperature no greater than the temperature of melt blending. The melt blend also incorporates a carboxyl and/or CO-functional ethylene polymer that compatibilizes the PVC and polyamide one with the other.
An alternate solution involves the use of laminar shaped articles. Laminar shaped articles are prepared from a melted, heterogeneous blend of two incompatible polymers and optionally a third polymer that is extended or stretched, such as in a blow molding or blown film operation. One of the incompatible polymers forms a continuous matrix phase. The other incompatible polymer forms a discontinuous distributed phase in which that polymer is present as a multitude of thin, substantially two dimensional, parallel and overlapping layers embedded in the continuous phase. Frequently, a third polymer is included as a compatibilizer to improve adhesion between the layers of the incompatible polymers so that the article has sufficient resistance to delamination. Laminar articles may be prepared using one extruder in a single operation, offering potentially significant advantages over multilayer structures in terms of reduced cost and complexity. Laminar compositions may also use less of the high-barrier polymer than ordinary melt blends to achieve the same level of barrier performance.
Laminar shaped articles prepared from a polyolefin and one or more polymers such as polyamide, polyester, or other polymers have been disclosed. See, e.g., U.S. Pat. Nos. 4,416,942, 4,444,817, 4,971,864, 5,085,816, 5,330,696, 5,399,619, 5,443,867, 5,641,833, and 5,700,412. See also U.S. Pat. Nos. 5,053,258, 5,712,043. and 6,576,181.
Laminar articles derived from a blend of polyolefin and nylon (polyamide) or nylon/polyvinyl alcohol blends have been used as fuel tanks for motor vehicles, providing a barrier minimizing the diffusion of hydrocarbons and oxygenated compounds in the fuel through the walls of the tank. U.S. Pat. No. 4,410,482 describes laminar articles of a polyolefin and a condensation polymer. EP0015556 describes laminar articles replacing nylon barrier with polyvinyl alcohol or ethylene/vinyl alcohol copolymers.
U.S. Pat. No. 4,950,513 describes a laminar article with improved barrier properties to oxygenated and hydrocarbon compounds blending about 60 to about 97 weight % polyolefin with a melt blend of about 2 to about 39 weight % nylon and a polyvinyl alcohol component, and using about 0.25 to about 12 weight % of an alkylcarboxyl-substituted polyolefin as a compatibilizer. The compatibilizer is prepared by grafting and has about 0.01 to about 20 weight % graft monomer.
U.S. Pat. No. 5,939,158 discloses a heterogeneous blend of (a) polyolefin with (b) a melt blend of a nylon and a polyvinyl alcohol component, and a compatibilizer (c), which is either i) a mixture of two different alkylcarboxyl-substituted polyolefins that serves as a compatibilizer; or ii) a small amount of alkylcarboxyl-substituted polyolefin with a small amount of grafted maleic anhydride moieties; or iii) a high weight percent of alkylcarboxyl-substituted polyolefin with a high graft level of grafted maleic anhydride moieties.